3.6 let

The let expression is a special form in Lisp that you will need
to use in most function definitions.

let is used to attach or bind a symbol to a value in such a way
that the Lisp interpreter will not confuse the variable with a
variable of the same name that is not part of the function.

To understand why the let special form is necessary, consider
the situation in which you own a home that you generally refer to as
`the house', as in the sentence, “The house needs painting.” If you
are visiting a friend and your host refers to `the house', he is
likely to be referring to his house, not yours, that is, to a
different house.

If your friend is referring to his house and you think he is referring
to your house, you may be in for some confusion. The same thing could
happen in Lisp if a variable that is used inside of one function has
the same name as a variable that is used inside of another function,
and the two are not intended to refer to the same value. The
let special form prevents this kind of confusion.